The present invention generally relates to the field of lighting, and more particularly to luminaires for indirect lighting which employ lens elements or other light transmissive media as a visible source of brightness.
With indirect lighting the light source is not seen directly. Rather, an interior space is illuminated by reflecting source light from interior wall surfaces. The result is a more even distribution of light and a reduction in contrast brightness that can produce glare. Because of this quality indirect lighting has become particularly advantageous in the open office environment where video display terminals (VDTs) are now prevalent and where uncomfortable glare on VDT screens produced by contrast brightness leads to operator fatigue and, some now believe, long term health problems.
A disadvantage of indirect lighting is that it tends to produce a dull lighting environment, sometimes called a "cloudy day effect." The dullness often associated with indirect lighting has heretofore been overcome by providing indirect lighting fixtures with visible lens elements that capture a portion of the source light and direct some of this light into normal viewing angles for the fixture, that is, angles below the plane of the fixture at which the fixture would be in the line of sight of persons within the illuminated space. The lens' visibility gives the observer a perception of source brightness and consequently the psychological advantage of being able to visually locate the light source.
But the effective use of such lens elements depends on the ability of the lighting designer to control the level of brightness and brightness uniformity on the observable lens surface. The desired result is a low brightness lens that provides just a hint of source indicative brightness to the observer, yet avoids the discomfort of excessive brightness and glare producing contrast brightness normally associated with direct lighting. Previously, lens brightness control has been achieved by designing lens prisms to direct a small amount of light only into normal viewing angles as disclosed in U.S. Pat. No. 4,390,930. Another patent, U.S. Pat. No. 4,698,734, discloses a special prismatic lens design that reduces troublesome hot spots, that is, small areas of intense brightness, that tend to appear on the faces of individual prisms at particular viewing angles.
However, lens design alone has proven inadequate to achieve acceptable lens brightness control and suppression of hot spots in all situations. This is particularly the case with lighting fixtures that use compact, high intensity fluorescent lamps, such as the Biax lamp manufactured by General Electric Company. Such lamps emit a large amount of light from a relatively small surface area as compared to more conventional fluorescent tubes, therefore making it difficult to control lens brightness by means of prism design. This problem is discussed in U.S. Pat. No. 4,939,627, which discloses to produce source brightness in a lens element by means of light reflected from surrounding ceiling or wall surfaces, referred to as a "secondary light source", rather than light received directly from the fluorescent lamp itself. Inducing lens brightness from such a secondary source of light eliminates hot spots and will generally produce greater uniformity in lens brightness. Nonetheless, such an approach has a distinct disadvantage: the brightness induced in the lens will be determined by the mounting environment for the lighting fixture and consequently will be difficult to predict. More specifically, the mounting or suspension distances for the lighting fixture will have a substantial effect on the lens brightness, as will the reflectivity of the surface which acts as the secondary source for the lens. For example, a fixture suspended very close to a ceiling having a highly reflective surface might produce a lens brightness that is uncomfortably high, while on the other hand a fixture suspended or mounted well below the ceiling surface, particularly one that has a low reflectivity, may produce no perceptible brightness at all. Thus, the indirect lighting fixture disclosed in U.S. Pat. No. 4,939,627 is not well adapted to a wide variety of installation environments. In particular, it is not well adapted to low mounting heights such as would be the case with an indirect fixture mounted slightly above eye level to an office furniture system.
Another disadvantage of the lens brightness control technique of U.S. Pat. No. 4,939,627 is that patterns, such as ceiling tile patterns, on ceiling or wall surfaces behind the fixture tend to be reflected through the lens. The result is that at certain viewing angles the lens will exhibit distinct shadow areas corresponding to these surface patterns. Also, the brightness of portions of the lens may vary depending on the angle at which the lens is viewed due to variations of brightness on the wall or ceiling surface. For example, in a suspended linear lighting fixture having linear lens elements extending the length of the fixture, the brightness of the ends of the lenses may diminish or become shadowy when viewed from a side angle due to the fact that the light pattern on the ceiling falls off rapidly beyond the ends of the fixture.
Thus, while the lens brightness control technique disclosed in U.S. Pat. No. 4,939,627 gives the designer the ability to achieve desired low brightness levels without hot spots on the lens using either conventional or compact high density fluorescent lamps, the problem of achieving desired brightness levels in varied mounting environments still exist as does the problem of achieving uniform brightness under all conditions and at a wide variety of viewing angles for the fixture.
The present invention overcomes the disadvantages of the indirect lighting fixture and system disclosed in U.S. Pat. No. 4,939,627 by providing a novel optical system that induces uniform low brightness in the lens elements of a lensed indirect lighting fixture, and does so in a way that is substantially unaffected by the fixture's mounting environment or the fixture's proximity to interior wall or ceiling surfaces. The invention permits the designer to easily establish any desired brightness level in the lens so the fixture can be adapted to a wide variety of architectural lighting environments. Additionally, in accordance with the invention, lens brightness can be made to be adjustable within the fixture such that brightness adjustments can be made after a fixture is installed. The fixture may be particularly adapted for use at low mounting heights such as on office furniture partitions where the intensity of ceiling reflected light is greatly diminished.